The present invention relates generally to WDM and DWDM communication systems, and more generally to an optical interleaver employed in such systems.
Optical wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have gradually become the standard backbone networks for fiber optic communication systems. WDM and DWDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM and DWDM technology.
One approach to increasing fiber optic capacity is to use more closely spaced channels. For example, at one point in time, 200 GHz spacing was common for optical channels. At that time optical components were designed to operate on 200 GHz spaced channels. As the state of the art improved, 100 GHz spacing was used for optical channels. Optical components were then designed to operate on 100 GHz spaced channels and devices designed to operate on 200 GHz spaced channels had to be replaced or modified to operate on the 100 GHz spaced channels. This upgrade requirement can be very expensive for parties with an extensive amount of fiber optic equipment that is already deployed.
An optical device that can be used for interfacing between different channel spacing schemes is known as an interleaver/deinterleaver, which is essentially an optical router that allows systems designed for operation at a wide channel spacing to be extended to systems designed for narrow channel spacings. In its simplest form, an interleaver combines two sets of channels into one densely packed set with half the channel spacing. Interleavers/deinterleavers are also used for other purposes, such as to add/drop channels at a node in such a way that one interleaver output adds/drops local channels while the other interleaver output forwards express channels to another node.
One example of an optical interleaver/deinterleaver is an interferometric interleaver/deinterleaver such as those shown in U.S. Pat. No. 6,281,977, one of which is reproduced in FIG. 1. The interleaver/deinterleaver 100 includes a ring resonator 110 and three directional couplers 112, 114, and 116. As shown in FIG. 2, ring resonator 110 and directional couplers 112 and 114 taken by themselves define a three port device with one input port and two output ports. The three port device has a transfer function that is equivalent to a Fabry-Perot resonator. The signals ar and at at the output ports of the device are equivalent to the reflected and transmitted signals of a Fabry-Perot resonator. The coupling coefficients of the directional couplers control the finesse of the cavity. The periodic spectral response of the three port device is determined by the total length of the ring through the following equation:
FSR=c/xcex94L
where c is the speed of light and xcex94L is the optical length through the ring.
Returning to the optical interleaver/deinterleaver of FIG. 1, the two outputs of the three port device that include ring resonator 110 are routed to coupler 116 to obtain two interleaved signals, aout1 and aout2. An interleaving function can be realized if the optical path length AC is equal to the optical path length BC, wherein the optical length is the product of the physical length and the refractive index.
While the performance characteristics of the aforementioned interleaver/deinterleaver, which include its isolation, transmission peak ripple, and effective pass-band width, are satisfactory for certain applications, it would nevertheless be desirable to provide an interferometric interleaver/deinterleaver that has improved performance characteristics.
In accordance with the present invention, a filtering device is provided that includes first and second optical couplers each having two inputs and two outputs. The inputs of the first coupler are adapted to receive input optical signals to be filtered and the outputs of the second coupler are adapted to provide filtered output optical signals. A first optical waveguide optically couples a first output of the first coupler to a first input of the second coupler. A second optical waveguide optically couples a second output of the first coupler to a second input of the second coupler. The filtering device also includes an optical resonator and a third optical coupler optically coupling the optical resonator to the second optical waveguide.
In accordance with one aspect of the invention, the optical resonator is a ring resonator.
In accordance with another aspect of the invention, the ring resonator has a circumferential optical path length greater than twice a difference in optical path length between the second optical waveguide and the first optical waveguide.
In accordance with yet another aspect of the invention, the ring resonator has a circumferential optical path length substantially equal to a difference in optical path length between the second optical waveguide and the first optical waveguide plus one half a center wavelength of a waveband in which the filtering device operates.
In accordance with another aspect of the invention, the first and second optical couplers are substantially 50:50 couplers.
In accordance with another aspect of the invention, the third optical coupler has a bar coupling ratio of about 0.18.
In accordance with another aspect of the invention, the first and second optical couplers are directional couplers.